In order to detect a moving object entering or approaching a particular site, it is common practice to detect any rise in the level of infrared energy in the detection region above a reference value. This system is called a passive type intrusion detection. The energy level rises by the addition of infrared energy radiated by a moving object within the detection region. Such detectors are usable not only for the intruder detecting system but also for automatic doors as a switch whereby a visiting guest is detected in advance.
The known detection system is liable to an erroneous response due to a sudden rise in the ambient temperature around the detection region by microwave noise, sunlight, or any other interference. In this specification, the "error" means the detection of a signal which is generated by any other object, other than a human such as a dog or cat. The erroneous detection leads to a false alarm.
Referring to FIG. 14, the known intruder detecting system will be described:
The known detection system includes a device for avoiding generating a false alarm. The error protective device comprises a detector 90 having a pair of infrared sensors 90a and 90b (three or more sensors can be used) wherein the sensors 90a and 90b are differentially connected to each other; that is, they are arranged in parallel or in series with opposite polarity. An optical system 91 is positioned for setting up detection regions A1 and A2 covering ordinary human heights. When a human intruder H or an animal M such as a dog passes through the detection regions A1 and A2, the passage through the two regions A1 and A2 takes place at a time interval. The intruder cannot cover the two regions at the same time. This differs point from ambient noise such as sunlight which covers the two regions at the same time. Because of the differential connection of the electrodes, the signals occurring in the regions A1 and A2 are mutually negated, thereby preventing a false signal from being generated. When a human intruder H passes through the detection regions A1 and A2, the human covers the whole space of the detection regions A1 and A2, thereby outputting a signal at a level higher than a reference level. If the moving object is a dog or any other non-human intruder, it only covers a lower part of the detection regions A1 and A2, thereby outputting a signal at a level lower than the reference level. Thus the production of a false alarm due to the passage of a moving object other than a human is avoided.
When a difference between the temperature of the moving object and the ambient temperature is small, an error can be easily avoided as shown in FIGS. 15(A) and 15(B). More specifically, the signal output by a human H is higher than the reference level as shown in FIG. 15(A) whereas the signal output by an animal M is lower than the reference level as shown in FIG. 15(B). When a difference between the temperature of the moving object and the ambient temperature is great, an error is likely to occur as shown in FIG. 16(B), because the signal output by a non-human object M like a dog is likely to exceed the reference level. As FIGS. 16(A) and 16(B) show, it is difficult to distinguish between an intruder and a dog.
In order to solve such problems, the inventor of the present invention has made an invention for which a Japanese Patent Application No. 5 (1993)-226058 is on file. As shown in FIGS. 9 and 10, the device of this prior invention includes sensors a and b to which infrared ray is led from detection region group Ah for a human intruder and group Am for an animal. The sensors a are connected to a first circuit c where all signals from the detection region group Ah are totaled, and the sensors b are connected to a second circuit d (FIG. 10, d') where all signals from the detection region group Am are totaled. Each circuit c and d is connected to a common arithmetic circuit e where a difference between the outputs from the region groups Ah and Am is calculated and a signal is generated.
In operation, when a human passes through the region groups Ah and Am, he or she covers the whole area of each region group Ah and the first circuit c generates a signal having a large peak, and the second circuit d generates a signal having a small peak because the signals arising from the passage of the human through the regions Am are mutually negated. Whereas, if a dog passes through the region group Ah, the generating signal has a small peak because the animal only covers a lower part of the detection region Ah. If a dog passes through the region group Am, the sensor b generates the same signal as the signal generated by the sensor a. In this way, there can be a difference between the signals occurring when a human passes through the detection region groups Ah and Am and when an animal passes therethrough.
FIG. 10 shows a modification in which the detection region group Am has plus and minus regions arranged differently from those shown in FIG. 9. In this embodiment the sensors a and b function in the same manner.
As FIGS. 9 and 10 show, the detection regions (plus and minus) in each group Ah and Am are spaced from one another. This arrangement sacrifices responsiveness. Preferably, the detection regions should be close to each other, or alternatively, they should overlap each other. However, if the detection regions are located close to each other or overlap each other, the sensor arrangement becomes complicated and costly.